First of all I have to give credit to cr0ybot and his instructable as that's where I first saw this mod.

This mod is a bit different. I wanted to kept the original look of the cartridge. The only tell sign is the mini USB port on the side.

Tools used:

Hobby drill
Soldering iron
Steel mini saw blade on a screw mandrel
Ruby grinding stone
Steel milling cutter
Modeling knife with flat blade attached
Custom 3.8mm Nintendo screw tool

Parts used:

Manhattan 2.5" IDE Hard drive enclosure
Old 2.5" 60GB IDE HDD (Hard Disk Drive)
NES Airwolf Cartridge

Step 1: Creating a Custom Tool & Opening the Cartridge

The cartridge is secured with the Nintendo 3.8mm screw. Since I could not get my hands on the specific tool tip I created my own custom tool.

I used a Pilot ball pen cap and with the steel milling cutter made it thinner in order for it to fit into the screw canal. I then used a lighter to soften the plastic and pressed it firmly into the canal, leaving it there for a few seconds to cool. After that the tool is ready for use!

Step 2: Stripping & Modifying Parts

For the IDE to USB converter the circuit board form a Manhattan External Enclosure was used. The circuit itself is glued to a plastic cover. By applying pressure on the glue in the direction of the plastic straps using a modeling knife, the glue snapped off the circuit board releasing the board from the cover.

The board has a DC power input socket, which in this case is not needed as the HDD powers up through USB without the need for external power. The socket was desoldered using the soldering iron.

Step 3: Modifying the Cartridge Casing

Attach the circuit board to the HDD and line it up onto the back part of the cartridge.

Align the setup and note down where you need to carve the plastic in order for the setup to rest into the back part of the cartridge. Then transfer the same marks onto the front part.

Carve using the steel milling cutter and ruby grinding stones.
Wear protective eye gear!

There are several other parts that also need to be carved out in order for the disk to fit into the cartridge.

Step 4: Cutting the Game Circuit Board

In order to stick as close to original look as possible, the game circuit board was kept. The specific board was bigger than other game boards so in order for it to fit, a piece needed to be removed.

Using the mini saw blade, cut the PCB so it fits in the cartridge along with the HDD.

Some circuits boards do not have the extra "row" that needed to be removed, so they might fit in into the cartridge "as is", in which case you will have a working NES cartridge hard drive enclosure!